1. Field of the Invention
The present invention concerns a polymer electrolyte, particularly a polymer electrolyte having a proton conductivity ability and, a membrane/electrode assembly, and a fuel cell.
2. Description of the Related Art
In recent years, lithium ion cells and fuel cells that can be utilized as power sources for portable equipments, etc. have been studied vigorously and, also for polymer electrolytes such as lithium ion conductive material, and proton conductive materials as members thereof have also been studied vigorously.
Further, it is preferred that a power source is a small size when the power is identical. Particularly, the fuel cells have been studied vigorously since they have advantageous features that they are quiet and exhaust gases are clean for internal combustion engines such as engines, as well as it has a possibility of providing high energy efficiency excellent over the internal combustion engines.
Generally, while sulfonic acid group containing perfluorocarbon polymers typically represented by Nafion (registered trade mark) have been used as the proton conductive material, since the materials have high ion conductivity but permeate also highly polar organic solvents such as methanol, the power is low in direct methanol type fuel cells. Further, for suppressing the permeation of methanol, only the aqueous methanol solution at a low concentration of about several % can be used as fuels, so that the energy density per unit weight or unit volume is lowered to result in a problem such that they cannot be applied to small-sized portable equipment uses. Further, it has also been demanded for a strength suitable to the process of manufacturing membrane/electrode assemblies and a sufficient durability in a case of use as fuel cells.
In recent years, there are many examples for the development of polymer electrolytes using highly rigid polymer materials and, studies have been made so far on the polymer electrolytes using resin materials of high solvent resistance among the polymer materials and polymer electrolytes mainly comprising sulfonated polyether ether ketone, sulfonated polysulfone, and sulfonated polyether ketone have been developed as disclosed in JP-A Nos. 6-49202, 6-93114, 8-20716, 9-245818, and 10-21943. However, since the sulfonic acid group is connected by way of a single bond to the aromatic ring of the polymer main chain, it involves a problem that sulfonic acid groups gradually cause dissociation at a high operation temperature to lower the cell performance. Further, also the proton conductivity is not at a sufficiently satisfactory level.
For solving the problems described above, polymer electrolytes in which sulfonic acid groups are introduced by way of a fluorinated alkylene group to aromatic rings of polymer main chains have been developed in JP-A Nos. 2004-2596, and 2005-314452. However, also the polymer electrolytes described above still leave a room for improvement with a view point of proton conductivity and durability and it has been demanded for further improvement of such performance intended for the practical use of the polymer electrolyte, particularly, a proton conductive film of a solid polymer fuel cell.